This invention relates to display collimator systems using a cathode ray tube for displaying electronically generated data and including standby reticle means for operation in the event of an electronic failure.
An optical collimator is intended to project to infinity the images of luminous objects which are arranged, for this purpose, in the focal area of an optical objective. As applied to aerial navigation, the luminous objects represent navigational information or information to assist with landing and are seen by the pilot through a semi-transparent combining glass superimposed on the outside world.
The luminous objects, also called reticles, are generated on the screen of the cathode-ray tube. There are other ways of producing reticles, by means of back-lighted masks or by means of equivalent arrangements such as those employing light-emitting diodes or optical fibres, which involve a large number of parts. In the case envisaged, a serious problem exists if the single display component formed by the cathode ray tube should break down in flight, since the pilot then no longer has any of the projected information available to him.
To deal with such drawbacks, the head-up-display is advantageously fitted with one or more standby reticles which are able to be substituted for the tube in the event of a display breakdown and which enable nevertheless a reduced amount of information useful for navigation purposes to be made available.
From the optical point of view, the standby reticles need to be situated in the focal plane in the same way as the screen of the cathode-ray tube. The collimator generally has a reflecting mirror arranged between the screen of the tube and the exit lens, and in addition the optical system is usually divided into two sets of lenses a first of which is arranged between the screen of the cathode-ray tube and the reflecting mirror and the second of which forms the exit optic. Consequently, there is only a small space between the screen of the tube and the first set of lenses and this makes it difficult to position a retractable standby reticle arrangement in this space. The solution generally adopted is to arrange the standby reticles at the sides in unoccupied spaces off the optical path traversed by the light rays coming from the screen of the cathode-ray tube. To allow operation in both modes, that is normal and standby, a semi-transparent dichroic mirror, or an equivalent optical arrangement based on prisms, is inserted on the optical axis running from the screen so that radiation from the tube will be transmitted with a high coefficient in transmission and that emitted by the standby reticles will be reflected with an equally high coefficient, it being understood that the two types of radiation lie in different spectral bands. The positions of the reticles and the mirror are so calculated that the plane of the reticles corresponds to a focal plane of the optical system. This technique has an advantage which derives from the fact that it does not require any mechanical arrangement for retracting the reticles. However, to render the reticles mobile in a predetermined direction, generally in elevation, it is necessary to have a mechanical arrangement, which is manually operated from the pilot's position, to shift the reticles with a suitable translatory movement or to control their shift. The pilot sets the rise, that is to say the desired value in elevation, manually, and this results in one or more of the standby reticles being shifted in their plane to the appropriate position in elevation.